Honeycomb structure and manufacturing method of the same

ABSTRACT

A honeycomb structure includes a tubular honeycomb structure body having porous partition walls to define and form a plurality of cells and an outer peripheral wall, and a pair of electrodes disposed on a side surface of the honeycomb structure body. An electrical resistivity of the honeycomb structure body is from 1 to 200 Ωcm, each of the pair of electrodes is formed into a band-like shape extending in an extending direction of the cells of the honeycomb structure body, one electrode in the pair of electrodes is disposed on a side opposite to the other electrode in the pair of electrodes via a center of the honeycomb structure body, the honeycomb structure body has a central region and an outer peripheral region, and an electrical resistivity of the central region is lower than an electrical resistivity of the outer peripheral region.

The present application is an application based on JP-2013-075391 filedon Mar. 29, 2013 with the Japanese Patent Office, the entire contents ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a honeycomb structure and amanufacturing method of the same. More particularly, the presentinvention relates to a honeycomb structure which is a catalyst carrierand also functions as a heater when a voltage is applied thereto andwhich can reduce energy when the voltage is applied thereto to purify anexhaust gas. Furthermore, the present invention relates to a honeycombstructure manufacturing method which can easily prepare such a honeycombstructure.

2. Description of Related Art

Heretofore, a product in which a catalyst is loaded onto a honeycombstructure made of cordierite has been used for a treatment of harmfulsubstances in an exhaust gas discharged from a car engine. Furthermore,it has also been known that a honeycomb structure formed from a sinteredsilicon carbide body is used for purification of the exhaust gas (e.g.,see Patent Document 1).

When the exhaust gas is treated by the catalyst loaded onto thehoneycomb structure, it is necessary to raise a temperature of thecatalyst to a predetermined temperature, but the catalyst temperature islow at start of the engine, which has caused a problem that the exhaustgas is not sufficiently purified.

Consequently, there has been investigated a method of disposing a heatermade of a metal on an upstream side of the honeycomb structure ontowhich a catalyst is loaded to raise the temperature of the exhaust gas(e.g., see Patent Document 2).

Furthermore, it has been suggested that a honeycomb structure body madeof a ceramic material may be used as “a heatable catalyst carrier”(e.g., see Patent Document 3).

[Patent Document 1] JP 4136319

[Patent Document 2] JP 2931362

[Patent Document 3] WO 2011/125815

SUMMARY OF THE INVENTION

When such a heater as described above is mounted and used on a car, apower source for use in an electric system of the car is used in common,for example, a power source of as high voltage as 200 V is used.However, the heater made of the metal has a low electric resistance.Therefore, when the power source of such a high voltage is used, acurrent excessively flows, which has caused a problem that the powersource circuit is damaged.

Furthermore, when the heater is made of metal, a catalyst cannot easilybe loaded onto the heater, even if the heater is processed into ahoneycomb structure. Therefore, it has been difficult to load thecatalyst integrally onto the heater.

Furthermore, a honeycomb structure described in Patent Document 3 ismade of a ceramic material having a predetermined electricalresistivity, and hence by energization, heat is evenly (without anyunevenness of temperature distribution) generated without any damages orthe like of an electric circuit. The honeycomb structure described inPatent Document 3 is excellent as an energization heat generation typecatalyst carrier. On the other hand, the current is allowed to evenlyflow through the whole honeycomb structure, and hence there still hasbeen room for improvement in that the energy efficiency is enhanced whena voltage is applied to the honeycomb structure to purify an exhaustgas.

The present invention has been developed in view of the above-mentionedproblem, and an object thereof is to provide a honeycomb structure whichis a catalyst carrier and also functions as a heater when a voltage isapplied thereto and which can reduce energy when the voltage is appliedthereto to purify an exhaust gas. Furthermore, an object of the presentinvention is to provide a honeycomb structure manufacturing method whichcan easily manufacture such a honeycomb structure.

To solve the above-mentioned problems, according to the presentinvention, the following honeycomb structure and manufacturing method ofthe honeycomb structure are provided.

According to a first aspect of the present invention, a honeycombstructure including a tubular honeycomb structure body having porouspartition walls to define and form a plurality of cells which becomethrough channels for a fluid and extend from an inflow end surface whichis an end surface on an inflow side of the fluid to an outflow endsurface which is an end surface on an outflow side of the fluid isprovided, and an outer peripheral wall positioned in the most outerperiphery; and a pair of electrodes disposed on a side surface of thehoneycomb structure body, wherein an electrical resistivity of thehoneycomb structure body is from 1 to 200 Ωcm, each of the pair ofelectrodes is formed into a band-like shape extending in an extendingdirection of the cells of the honeycomb structure body, and in a crosssection perpendicular to the cell extending direction, one electrode inthe pair of electrodes is disposed on a side opposite to the otherelectrode in the pair of electrodes via a center of the honeycombstructure body, the honeycomb structure body is constituted of an outerperipheral region including the side surface and a central region as aregion around the center which excludes the outer peripheral region, andan electrical resistivity of the central region is lower than anelectrical resistivity of the outer peripheral region.

According to a second aspect of the present invention, the honeycombstructure according to the first aspect is provided, wherein anelectrical resistivity of a material constituting the central region islower than an electrical resistivity of a material constituting theouter peripheral region.

According to a third aspect of the present invention, the honeycombstructure according to the first or second aspects is provided, whereinthe honeycomb structure body and the electrodes are made of a materialincluding silicon carbide.

According to a fourth aspect of the present invention, the honeycombstructure according to any one of the first to third aspects isprovided, wherein in the cross section perpendicular to the cellextending direction, a length of a current path is 1.6 times or less adiameter of the honeycomb structure body.

According to a fifth aspect of the present invention, the honeycombstructure according to any one of the first to fourth aspects isprovided, wherein the central region has a boundary region in a boundaryportion between the central region and the outer peripheral region, andthe boundary region is a region where the electrical resistivitygradually changes so that the electrical resistivity lowers toward thecloser boundary portion to the outer peripheral region.

According to a sixth aspect of the present invention, the manufacturingmethod of a honeycomb structure having a formed honeycomb body preparingstep of extrusion-forming a forming raw material containing a ceramicraw material is provided, to prepare a formed honeycomb body havingpartition walls to define and form a plurality of cells which becomethrough channels for a fluid and extend from one end surface to theother end surface and an outer peripheral wall positioned in the mostouter periphery; a dried honeycomb body preparing step of drying theformed honeycomb body to prepare a dried honeycomb body; a firedhoneycomb body preparing step of firing the dried honeycomb body toprepare a fired honeycomb body; a preparing step of the fired honeycombbody with unfired electrodes in which an electrode forming raw materialcontaining a ceramic raw material is applied to a side surface of thefired honeycomb body and dried to form the unfired electrodes, therebypreparing the fired honeycomb body with the unfired electrodes; and ahoneycomb structure preparing step of firing the fired honeycomb bodywith the unfired electrodes to prepare the honeycomb structure, whereinin the fired honeycomb body preparing step, the dried honeycomb body isfired in a state where a plurality of particles containing, as a maincomponent, cordierite, carbon or a mixture of these components are incontact with a side surface of the dried honeycomb body (a firsthoneycomb structure manufacturing method).

According to an eighteenth aspect of the present invention, amanufacturing method of a honeycomb structure having a formed honeycombbody preparing step of extrusion-forming a forming raw materialcontaining a ceramic raw material is provided, to prepare a formedhoneycomb body having partition walls to define and form a plurality ofcells which become through channels for a fluid and extend from one endsurface to the other end surface and an outer peripheral wall positionedin the most outer periphery; a dried honeycomb body preparing step ofdrying the formed honeycomb body to prepare a dried honeycomb body; apreparing step of the dried honeycomb body with unfired electrodes inwhich an electrode forming raw material containing a ceramic rawmaterial is applied to a side surface of the dried honeycomb body anddried to form the unfired electrodes, thereby preparing the driedhoneycomb body with the unfired electrodes; and a honeycomb structurepreparing step of firing the dried honeycomb body with the unfiredelectrodes to prepare the honeycomb structure, wherein in the honeycombstructure preparing step, the dried honeycomb body with the unfiredelectrodes is fired in a state where a plurality of particlescontaining, as a main component, cordierite, carbon or a mixture ofthese components are in contact with the side surface of the driedhoneycomb body with the unfired electrodes (a second honeycomb structuremanufacturing method).

In the honeycomb structure of the present invention, the electricalresistivity of the central region is lower than the electricalresistivity of the outer peripheral region. Therefore, when a voltage isapplied to the honeycomb structure, a large amount of current flows tothe inflow side region. On the other hand, the exhaust gas flowing intothe honeycomb structure from the inflow end surface thereof flows moreto the central region of a honeycomb structure body. Therefore, moresubstances to be treated flow in the central region of the honeycombstructure body than in the outer peripheral region of the honeycombstructure body. As described above, in the honeycomb structure of thepresent invention, more current flows in the central region where “theflow of the substances to be treated is large”, and less current flowsin the outer peripheral region where “the flow of the substances to betreated is small”. Consequently, in the honeycomb structure of thepresent invention, the current allowed to flow by the application of thevoltage can effectively be used for a treatment of the substances to betreated in the exhaust gas. Furthermore, a useless current can beinhibited from flowing through a region including less substances to betreated.

In the first honeycomb structure manufacturing method of the presentinvention, in the fired honeycomb body preparing step, the driedhoneycomb body is fired in a state where a plurality of particlescontaining, as a main component, cordierite, carbon or a mixture ofthese components are in contact with the side surface of the driedhoneycomb body. Therefore, when the dried honeycomb body is fired, metalsilicon contained in the dried honeycomb body is discharged to theoutside by the presence of “cordierite, carbon, or both of thesecomponents”. Furthermore, the honeycomb structure body “where theelectrical resistivity of the central region is lower than theelectrical resistivity of the outer peripheral region” is accordinglyeasily formed.

Here, the plurality of particles containing, as the main component,“cordierite, carbon or the mixture of these components” are referred toas “specific particles”. In the second honeycomb structure manufacturingmethod of the present invention, in the honeycomb structure preparingstep, the dried honeycomb body with unfired electrodes is fired in astate where “the specific particles” are in contact with a side surfaceof the dried honeycomb body with the unfired electrodes. Therefore, whenthe dried honeycomb body with the unfired electrodes is fired, metalsilicon contained in the dried honeycomb body with the unfiredelectrodes is discharged to the outside by the presence of “cordierite,carbon or both of these components”. Furthermore, the honeycombstructure body “where the electrical resistivity of the central regionis lower than the electrical resistivity of the outer peripheral region”is accordingly easily formed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view schematically showing one embodiment of thehoneycomb structure of the present invention;

FIG. 2 is a schematic view showing a cross section parallel to the cellextending direction in the one embodiment of the honeycomb structure ofthe present invention;

FIG. 3 is a schematic view showing a cross section perpendicular to thecell extending direction in the one embodiment of the honeycombstructure of the present invention;

FIG. 4 is a schematic view showing a cross section perpendicular to thecell extending direction in the one embodiment of the honeycombstructure of the present invention;

FIG. 5 is a schematic view showing a cross section perpendicular to thecell extending direction in another embodiment of the honeycombstructure of the present invention;

FIG. 6 is a disc formed by cutting the honeycomb structure of Example 1in round slices; and

FIG. 7 shows a rod-like sample cut out from the honeycomb structure ofExample 1.

DETAILED DESCRIPTION OF THE INVENTION

Next, embodiments of the present invention will be described in detailwith reference to the drawings. It should be understood that the presentinvention is not limited to embodiments in the following and thatchanges, improvements and the like can suitably be added on the basis ofordinary knowledge of a person skilled in the art without departing fromthe gist of the present invention.

(1) Honeycomb Structure:

As shown in FIG. 1 to FIG. 4, one embodiment of a honeycomb structure ofthe present invention includes a tubular honeycomb structure body 4 anda pair of electrodes 21, 21 disposed on a side surface 5 of thehoneycomb structure body 4. The honeycomb structure body 4 has porouspartition walls 1 to define and form a plurality of cells 2 which becomethrough channels for a fluid and extend from an inflow end surface 11which is an end surface on an inflow side of the fluid to an outflow endsurface 12 which is an end surface on an outflow side of the fluid, andan outer peripheral wall 3 positioned in the most outer periphery.Furthermore, the electrical resistivity of the honeycomb structure body4 is from 1 to 200 Ωcm. Furthermore, each of the pair of electrodes 21,21 is formed into a band-like shape extending in an extending directionof the cells 2 of the honeycomb structure body 4. Furthermore, in across section perpendicular to the extending direction of the cells 2,one electrode 21 in the pair of electrodes 21, 21 is disposed on theside opposite to the other electrode 21 in the pair of electrodes 21, 21via the center O of the honeycomb structure body 4. Furthermore, thehoneycomb structure body 4 is constituted of an outer peripheral region7 including the side surface 5 and a central region 6 as a region aroundthe center which excludes the outer peripheral region 7. Furthermore,the electrical resistivity of the central region 6 is lower than anelectrical resistivity of the outer peripheral region 7. FIG. 1 is aperspective view schematically showing one embodiment (honeycombstructure 100) of the honeycomb structure of the present invention. FIG.2 is a schematic view showing a cross section parallel to the cellextending direction in the one embodiment of the honeycomb structure ofthe present invention. FIG. 3 is a schematic view showing a crosssection perpendicular to the cell extending direction in the oneembodiment of the honeycomb structure of the present invention. FIG. 4is a schematic view showing a cross section perpendicular to the cellextending direction in the one embodiment of the honeycomb structure ofthe present invention. It is to be noted that in FIG. 3, the partitionwalls are omitted. Moreover, in FIG. 4, the partition walls are omitted,and furthermore, the central region and the outer peripheral region arenot shown.

As described above, in the honeycomb structure 100 of the presentembodiment, the electrical resistivity of the honeycomb structure body 4is from 1 to 200 Ωcm. Therefore, even when a current is allowed to flowby using a power source of a high voltage, the current does notexcessively flow, so that the honeycomb structure can suitably be usedas a heater. Furthermore, in the honeycomb structure 100 of the presentembodiment, each of the pair of electrodes 21, 21 is formed into theband-like shape extending in the extending direction of the cells 2 ofthe honeycomb structure body 4. Furthermore, in the cross sectionperpendicular to the extending direction of the cells 2, the oneelectrode 21 in the pair of electrodes 21, 21 is disposed on the sideopposite to the other electrode 21 in the pair of electrodes 21, 21 viathe center of the honeycomb structure body 4. Therefore, it is possibleto inhibit an unevenness of a temperature distribution in the honeycombstructure body 4 when the voltage is applied between the pair ofelectrodes 21, 21. It is to be noted that the temperature of the centralregion 6 may be different from the temperature of the outer peripheralregion 7. “The unevenness of the temperature distribution in thehoneycomb structure body 4” means that the temperature of some portionof the honeycomb structure body 4 is locally high or locally low.

Furthermore, in the honeycomb structure 100 of the present embodiment,the electrical resistivity of the central region is lower than theelectrical resistivity of the outer peripheral region, and hence whenthe voltage is applied to the honeycomb structure body, a large amountof current flows in the inflow side region. Therefore, in the honeycombstructure of the present embodiment, more current flows in the centralregion where “the flow of substances to be treated is large”, and lesscurrent flows in the outer peripheral region where “the flow of thesubstances to be treated is small”. Consequently, in the honeycombstructure of the present invention, the current allowed to flow by theapplication of the voltage can effectively be used for the treatment ofthe substances to be treated in the exhaust gas. Furthermore, a uselesscurrent can be inhibited from flowing through a region including lesssubstances to be treated.

When the honeycomb structure body is divided into the inflow side regionwhich includes the inflow end surface and which is a region on theinflow side of the fluid and an outflow side region which is a regionexcluding the inflow side region, the electrical resistivity of thematerial constituting the inflow side region is preferably lower thanthe electrical resistivity of the material constituting the outflow sideregion.

Here, when “in the cross section perpendicular to the extendingdirection of the cells 2, the one electrode 21 in the pair of electrodes21, 21 is disposed on the side opposite to the other electrode 21 in thepair of electrodes 21, 21 via the center O of the honeycomb structurebody 4”, the following is meant. In the cross section perpendicular tothe extending direction of the cells 2, “a line segment connecting thecentral point of the one electrode 21 (central point in “the peripheraldirection of the honeycomb structure body 4”) to the center O of thehoneycomb structure body 4” is to be a first line segment. In the crosssection perpendicular to the extending direction of the cells 2, “a linesegment connecting the central point of the other electrode 21 (centralpoint in “the peripheral direction of the honeycomb structure body 4”)to the center O of the honeycomb structure body 4” is to be a secondline segment. Then the pair of electrodes 21, 21 are disposed in thehoneycomb structure body 4 in such a positional relation that an angle βformed by the first line segment and the second line segment (an anglearound “the center O” (see FIG. 4)) is in a range of 170° to 190°.Moreover, as shown in FIG. 4, “a central angle α of the electrode 21” isan angle formed by two line segments connecting both ends of theelectrode 21 to the center O of the honeycomb structure body 4 in thecross section perpendicular to the cell extending direction.Furthermore, “the central angle α of the electrode 21” can also bedescribed as follows. In the cross section perpendicular to the cellextending direction, “a line segment connecting one end portion of theelectrode 21 to the center O” is to be a third line segment. In thecross section perpendicular to the cell extending direction, “a linesegment connecting the other end portion of the electrode 21 to thecenter O” is to be a fourth line segment. Then “the central angle α ofthe electrode 21” is an inner angle of a portion of the center O in ashape formed by “the electrode 21”, the third line segment, and thefourth line segment (e.g., a fan shape) in the cross sectionperpendicular to the cell extending direction.

A value of a ratio of the electrical resistivity of the outer peripheralregion 7 to the electrical resistivity of the central region 6 (theouter peripheral region/the central region) is preferably from 1.0 to10.0 and further preferably from 1.1 to 5.0. When “the outer peripheralregion/the central region” is lower than 1.0, the current flows morethan necessary sometimes. When “the outer peripheral region/the centralregion” is higher than 10.0, the heat generation may be uneven.Furthermore, there is a fear that cracks are disadvantageously generatedcaused by energization. The electrical resistivity is a value measuredby a four-terminal method.

In the honeycomb structure 100 of the present embodiment, the centralregion 6 is a region positioned in a central portion (a portion whichdoes not include the outer periphery) of the honeycomb structure body 4in the cross section perpendicular to the cell extending direction.Furthermore, the central region 6 is a region positioned so as to extendthrough the central portion of the honeycomb structure body from theinflow end surface 11 to the outflow end surface 12. As shown in FIG. 1,when the honeycomb structure body 4 has a cylindrical shape, the centralregion 6 preferably also has a cylindrical shape.

In the honeycomb structure 100 of the present embodiment, a distancefrom the center O to an outer periphery of the central region 6 (centralregion radius) is preferably from 40 to 90% of a distance from thecenter O to an outer periphery of the outer peripheral region 7 (outerperipheral region radius) in the cross section perpendicular to the cellextending direction. Moreover, the central region radius is furtherpreferably from 50 to 80% of the outer peripheral region radius. Whenthe percentage is smaller than 40%, the effect that “the current allowedto flow by the application of the voltage can effectively be used forthe treatment of the substances to be treated in the exhaust gas” maydeteriorate. When the percentage is larger than 90%, the current mayflow more than necessary through the whole structure during theapplication of the voltage.

In the honeycomb structure 100 of the present embodiment, the electricalresistivity of the material constituting the central region ispreferably lower than the electrical resistivity of the materialconstituting the outer peripheral region. In consequence, the electricalresistivities of the central region and the outer peripheral region caneasily be regulated simply by changing the raw material used in thepreparation of the honeycomb structure.

In the honeycomb structure 100 of the present embodiment, the honeycombstructure body 4 and the electrodes 21 are preferably made of a materialincluding silicon carbide. An example of the material including siliconcarbide is a material containing a silicon-silicon carbide compositematerial, silicon carbide or the like as a main component. In theseexamples, the material containing the silicon-silicon carbide compositematerial as the main component is further preferable. Furthermore, thematerial constituting the honeycomb structure body 4 and the electrodes21 is especially preferably a material containing 95 mass % or more ofthe silicon-silicon carbide composite material. In the presentdescription, when “the material is the silicon-silicon carbide compositematerial”, it is meant that the material contains 95 mass % or more of“the silicon-silicon carbide composite material”. Here, “the maincomponent” is a component contained as much as 90 mass % or more in thewhole material. The silicon-silicon carbide composite material is amaterial in which a plurality of silicon carbide particles are bound bymetal silicon. The silicon-silicon carbide composite material ispreferably porous, because “the plurality of silicon carbide particlesare bound by metal silicon so that pores are formed among the siliconcarbide particles”. By use of such a material, the electricalresistivity of the honeycomb structure body can be from 1 to 200 Ωcm.The electrical resistivity of the honeycomb structure body is a value at400° C. Furthermore, when the honeycomb structure body 4 and theelectrodes 21 contain the silicon carbide particles and silicon as themain components, the components of the electrodes 21 and the componentsof the honeycomb structure body 4 are the same components or closecomponents. Therefore, thermal expansion coefficients of the electrodes21 and the honeycomb structure body 4 have the same value or closevalues. Furthermore, since the materials are the same or close to eachother, the bonding strength between the electrode 21 and the honeycombstructure body 4 heightens. Therefore, even when heat stress is appliedto the honeycomb structure, the electrodes 21 can be prevented frombeing peeled from the honeycomb structure body 4, or the bonding portionbetween the electrode 21 and the honeycomb structure body 4 can beprevented from being damaged.

Moreover, when the material of the central region 6 and the outerperipheral region 7 is the silicon-silicon carbide composite material,the central region 6 has “a higher content ratio of metal silicon in thesilicon-silicon carbide composite material than the outer peripheralregion 7”, and preferably accordingly has a low electrical resistivity.

When the material of the central region 6 is the silicon-silicon carbidecomposite material, the content ratio of metal silicon in thesilicon-silicon carbide composite material is preferably from 10 to 50mass % and further preferably from 20 to 40 mass %. When the contentratio is smaller than 10 mass %, the electrical resistivity of thecentral region 6 may be excessively high. When the content ratio islarger than 50 mass %, the electrical resistivity of the central region6 may be excessively low.

When the material of the outer peripheral region 7 is thesilicon-silicon carbide composite material, the content ratio of metalsilicon in the silicon-silicon carbide composite material is preferablyfrom 10 to 50 mass % and further preferably from 10 to 30 mass %. Whenthe content ratio is smaller than 10 mass %, the electrical resistivityof the outer peripheral region 7 may be excessively high. When thecontent ratio is larger than 50 mass %, the electrical resistivity ofthe outer peripheral region 7 may be excessively low.

In the honeycomb structure of the present invention, the central region6 preferably has a boundary region 8 in a boundary portion between thecentral region and the outer peripheral region 7 (see FIG. 5).Furthermore, the boundary region 8 is preferably a region where theelectrical resistivity gradually changes so that the electricalresistivity is lower toward the closer boundary portion to the outerperipheral region 7. The boundary region 8 is a region “from “an end 8 aon the side of the central region 6” to “the boundary 8 b with the outerperipheral region 7”” where the electrical resistivity changes at aratio of 0.1 Ωcm/cm or more. That is, in the boundary region 8 “from theend on the side of the central region 6 to the boundary with the outerperipheral region 7” in the cell extending direction, the change ratioof the electrical resistivity is 0.1 Ωcm/cm or more. In the boundaryregion 8 “from the end on the side of the central region 6 to theboundary with the outer peripheral region 7” in the cell extendingdirection, the change ratio of the electrical resistivity is preferablyfrom 0.1 to 10 Ωcm/cm. Furthermore, the change ratio of the electricalresistivity is further preferably from 0.5 to 10 Ωcm/cm. When the changeratio is larger than 10 Ωcm/cm, there is a fear that currentconcentration occurs in the boundary region to cause cracks or shortcircuit. The thickness of the boundary region 8 (value obtained bysubtracting the distance between the center O and “the end 8 a on theside of the central region” from the distance between the center O and“the boundary 8 b with the outer peripheral region”) is preferably from3 to 30% of “the distance from the center O to the outer periphery” ofthe honeycomb structure body. Furthermore, the thickness of the boundaryregion 8 is further preferably from 3 to 20% of “the distance from thecenter O to the outer periphery” of the honeycomb structure body. Whenthe percentage is smaller than 3%, there is the fear that the currentconcentration occurs in the boundary region to cause the cracks or theshort circuit. When the material forming the central region 6 is “thesilicon-silicon carbide composite material”, the boundary region 8preferably has the following constitution. That is, the boundary region8 is preferably formed so that “the content ratio of metal silicongradually changes “from the end 8 a on the side of the central region 6to the boundary 8 b with the outer peripheral region 7””, whereby theelectrical resistivity changes. It is to be noted that when the centralregion 6 has the boundary region 8, the electrical resistivity of thecentral region 6 is the electrical resistivity of the whole centralregion 6 including the boundary region 8. FIG. 5 is a front viewschematically showing another embodiment of the honeycomb structure ofthe present invention (honeycomb structure 200). The honeycomb structure200 of the present embodiment is preferably similar to the oneembodiment of the honeycomb structure of the present invention(honeycomb structure 100 (see FIG. 1)), except that the honeycombstructure 200 has the boundary region 8.

In the honeycomb structure 100 of the present embodiment, as shown inFIG. 1 to FIG. 4, the pair of electrodes 21, 21 are disposed on the sidesurface 5 of the honeycomb structure body 4. The honeycomb structure 100of the present embodiment generates the heat when the voltage is appliedbetween the pair of electrodes 21, 21. The voltage to be applied ispreferably from 12 to 900 V and further preferably from 64 to 600 V.

In the honeycomb structure 100 of the present embodiment, when amaterial forming the honeycomb structure body 4 is “the silicon-siliconcarbide composite material”, an average particle diameter of the siliconcarbide particles (aggregates) constituting the honeycomb structure body4 is preferably from 3 to 50 μm. Furthermore, the average particlediameter of the silicon carbide particles (aggregates) constituting thehoneycomb structure body 4 is further preferably from 3 to 40 μm. Whenthe average particle diameter of the silicon carbide particlesconstituting the honeycomb structure body 4 is in such a range, theelectrical resistivity of the honeycomb structure body 4 at 400° C. canbe from 1 to 200 Ωcm. When the average particle diameter of the siliconcarbide particles is smaller than 3 μm, the electrical resistivity ofthe honeycomb structure body 4 may be large. When the average particlediameter of the silicon carbide particles is larger than 50 μm, theelectrical resistivity of the honeycomb structure body 4 may be small.Furthermore, when the average particle diameter of the silicon carbideparticles is larger than 50 μm, the die for extrusion-forming may beclogged with the forming raw material during the extrusion-forming ofthe formed honeycomb body. The average particle diameter of the siliconcarbide particles is a value measured by laser diffraction method.

A porosity of the partition walls 1 of the honeycomb structure body 4 ispreferably from 35 to 60% and further preferably from 35 to 45%. Whenthe porosity is smaller than 35%, deformation during firing may bedisadvantageously large. When the porosity is in excess of 60%, thestrength of the honeycomb structure may deteriorate. The porosity is avalue measured by mercury porosimeter.

An average pore diameter of the partition walls 1 of the honeycombstructure body 4 is preferably from 2 to 15 μm and further preferablyfrom 4 to 8 μm. When the average pore diameter is smaller than 2 μm, theelectrical resistivity may be excessively large. When the average porediameter is larger than 15 μm, the electrical resistivity may beexcessively small. The average pore diameter is a value measured bymercury porosimeter.

In the honeycomb structure 100 of the present embodiment, a thickness ofthe partition walls 1 of the honeycomb structure body 4 is preferablyfrom 50 to 200 μm and further preferably from 70 to 180 μm. Thethickness of the partition walls is in such a range, whereby when thehoneycomb structure 100 is used as a catalyst carrier and catalyst isloaded thereonto, the pressure loss during the flowing of an exhaust gascan be prevented from being excessively large. When the thickness of thepartition walls is smaller than 50 μm, the strength of the honeycombstructure may deteriorate. When the thickness of the partition walls islarger than 200 μm, the pressure loss during the flowing of the exhaustgas may be large, in the case where the honeycomb structure 100 is usedas a catalyst carrier and catalyst is loaded thereonto.

In the honeycomb structure 100 of the present embodiment, a cell densityof the honeycomb structure body 4 is preferably from 40 to 150 cells/cm²and further preferably from 70 to 100 cells/cm². When the cell densityis in such a range, the purification performance of the catalyst can beheightened while the pressure loss during the flowing of the exhaust gasis small. When the cell density is lower than 40 cells/cm², the catalystloading area may be decreased. When the cell density is higher than 150cells/cm², the pressure loss during the flowing of the exhaust gas maybe large, in the case where the honeycomb structure 100 is used as acatalyst carrier with catalyst loaded thereonto.

There is not any special restriction on a shape of the honeycombstructure 100 (shape of the honeycomb structure body 4) of the presentembodiment, and examples of the shape include a tubular shape with abottom surface having a round shape (cylindrical shape), a tubular shapewith a bottom surface having an oval shape, a tubular shape with abottom surface having a polygonal shape (quadrangular shape, pentangularshape, hexagonal shape, heptagonal shape, octagonal shape or the like),and the like. Furthermore, as to a size of the honeycomb structure(honeycomb structure body), an area of the bottom surface is from 2000to 20000 mm² and further preferably from 4000 to 10000 mm². Furthermore,a length of the honeycomb structure (honeycomb structure body) in acentral axis direction is preferably from 50 to 200 mm and furtherpreferably from 75 to 150 mm. Moreover, a diameter of each end surfaceof the honeycomb structure body is preferably three times or less,further preferably from 0.5 to 2.5 times, and especially preferably from0.8 times to twice a length of the honeycomb structure body in the cellextending direction. When the diameter of the end surface of thehoneycomb structure body is in excess of three times the length of thehoneycomb structure body in the cell extending direction, the honeycombvolume may be small, and hence it may not be possible to load such anamount of the catalyst as to sufficiently exert the purificationperformance of the exhaust gas.

Furthermore, in the honeycomb structure 100 of the present embodiment, athickness of the outer peripheral wall 3 constituting the most outerperiphery of the honeycomb structure body 4 is preferably from 0.1 to 2mm. When the thickness is smaller than 0.1 mm, the strength of thehoneycomb structure 100 may deteriorate. When the thickness is largerthan 2 mm, the area of each partition wall onto which the catalyst is tobe loaded may decrease.

In the honeycomb structure 100 of the present embodiment, a shape of thecells 2 in the cross section perpendicular to the extending direction ofthe cells 2 is preferably a quadrangular shape, a hexagonal shape, anoctagonal shape, or any combination of these shapes. When the cell shapeis in such a shape, the pressure loss during the flowing of the exhaustgas through the honeycomb structure 100 is small, and the purificationperformance of the catalyst is excellent.

As shown in FIG. 1 to FIG. 4, in the honeycomb structure 100 of thepresent embodiment, each of the pair of electrodes 21, 21 is formed into“the band-like shape” extending in the extending direction of the cells2 of the honeycomb structure body 4. Furthermore, in the cross sectionperpendicular to the extending direction of the cells 2, the oneelectrode 21 in the pair of electrodes 21, 21 is disposed on the sideopposite to the other electrode 21 in the pair of electrodes 21, 21 viathe center O of the honeycomb structure body 4. As such, in thehoneycomb structure 100 of the present embodiment, the electrode 21 isformed into the band-like shape, a longitudinal direction of theelectrode 21 is the extending direction of the cells 2 of the honeycombstructure body 4, and the pair of electrodes 21, 21 are disposed on theopposite sides via the center O of the honeycomb structure body 4.Therefore, when the voltage is applied between the pair of electrodes21, 21, it is possible to inhibit the unevenness of the current flowingthrough each of the central region 6 and the outer peripheral region 7.In consequence, it is possible to inhibit the unevenness of the heatgeneration in each of the central region 6 and the outer peripheralregion 7.

Furthermore, in the cross section perpendicular to the extendingdirection of the cells 2, an angle of 0.5 times the central angle α ofeach of the electrodes 21, 21 (angle θ of 0.5 times the central angle α)is preferably from 15 to 65°. Furthermore, in the cross sectionperpendicular to the extending direction of the cells 2, an upper limitvalue of “the angle θ of 0.5 times the central angle α” of each of theelectrodes 21, 21 is preferably 60° and further preferably 55°.Furthermore, in the cross section perpendicular to the extendingdirection of the cells 2, a lower limit value of “the angle θ of 0.5times the central angle α” of each of the electrodes 21, 21 ispreferably 20° and further preferably 30°. Furthermore, “the angle θ of0.5 times the central angle α” of the one electrode 21 preferably has asize of 0.8 to 1.2 times and further preferably a size of 1.0 time (thesame size) to “the angle θ of 0.5 times the central angle α” of theother electrode 21. In consequence, when the voltage is applied betweenthe pair of electrodes 21, 21, it is possible to inhibit the unevennessof the current flowing through each of the central region 6 and theouter peripheral region 7. In consequence, it is possible to inhibit theunevenness of the heat generation in each of the central region 6 andthe outer peripheral region 7.

In the honeycomb structure 100 of the present embodiment, the electricalresistivity of the electrode 21 is preferably lower than the electricalresistivity of the central region 6 of the honeycomb structure body 4.Furthermore, the electrical resistivity of the electrode 21 is furtherpreferably 20% or less and especially preferably from 1 to 10% of theelectrical resistivity of the central region 6 of the honeycombstructure body 4. When the electrical resistivity of the electrode 21 is20% or less of the electrical resistivity of the central region 6 of thehoneycomb structure body 4, the electrode 21 more effectively functionsas an electrode.

A thickness of the electrode 21 is preferably from 0.01 to 5 mm andfurther preferably from 0.01 to 3 mm. In such a range, the heat canevenly be generated in each of the central region 6 and the outerperipheral region 7. When the thickness of the electrode 21 is smallerthan 0.01 mm, electric resistance heightens, and hence the heat cannotevenly be generated. When the thickness is larger than 5 mm, eachelectrode may be damaged during canning.

As shown in FIG. 1 and FIG. 2, in the honeycomb structure 100 of thepresent embodiment, each of the pair of electrodes 21, 21 is formed intothe band-like shape extending in the extending direction of the cells 2of the honeycomb structure body 4 and “extending between both the endportions (between both the end surfaces 11 and 12)”. As such, in thehoneycomb structure 100 of the present embodiment, the pair ofelectrodes 21, 21 are disposed so as to extend between both end portionsof the honeycomb structure body 4. In consequence, it is possible tomore effectively inhibit the unevenness of the current flowing througheach of the central region 6 and the outer peripheral region 7 when thevoltage is applied between the pair of electrodes 21, 21. Here, when“the electrode 21 is formed (disposed) so as to extend between both endportions of the honeycomb structure body 4”, the following is meant.That is, it is meant that one end portion of the electrode 21 comes incontact with one end portion (one end surface) of the honeycombstructure body 4, and the other end portion of the electrode 21 comes incontact with the other end portion (the other end surface) of thehoneycomb structure body 4.

On the other hand, it is also preferable that at least one end portionof the electrode 21 in “the extending direction of the cells 2 of thehoneycomb structure body 4” does not come in contact with (does notreach) the end portion (the end surface) of the honeycomb structure body4. In consequence, heat shock resisting properties of the honeycombstructure can be enhanced.

In the honeycomb structure 100 of the present embodiment, for example,as shown in FIG. 1 to FIG. 4, the electrode 21 has such a shape asobtained by curving a planar rectangular member along an outer peripheryof a cylindrical shape. Here, a shape obtained when the curved electrode21 is deformed into a planar member which is not curved will be referredto as “the planar shape” of the electrode 21. The above-mentioned“planar shape” of the electrode 21 shown in FIG. 1 to FIG. 4 is arectangular shape. Furthermore, “an outer peripheral shape of theelectrode” means “the outer peripheral shape in the planar shape of theelectrode”.

In the honeycomb structure 100 of the present embodiment, the outerperipheral shape of the band-like electrode may be a shape in whichcorner portions of the rectangular shape are curved. According to such ashape, the heat shock resisting properties of the honeycomb structurecan be enhanced. It is also preferable that the outer peripheral shapeof the band-like electrode is a shape in which the corner portions ofthe rectangular shape are linearly chamfered. According to such a shape,the heat shock resisting properties of the honeycomb structure can beenhanced.

In the honeycomb structure 100 of the present embodiment, in the crosssection perpendicular to the cell extending direction, a length of thecurrent path is preferably 1.6 times or less the diameter of thehoneycomb structure body. In excess of 1.6 times, energy isdisadvantageously unnecessarily consumed. Here, “the current path” is apath through which the current flows. Furthermore, “the length of thecurrent path” is a length of 0.5 times a length of “the outer periphery”where the current flows, in “the cross section perpendicular to the cellextending direction” of the honeycomb structure body. This means themaximum length in “the path through which the current flows” in “thecross section perpendicular to the cell extending direction” of thehoneycomb structure body. When a concave or a convex is formed in theouter periphery or when a slit opened in the outer periphery is formedin the honeycomb structure body, “the length of the current path” is avalue measured along the surface of the concave/convex or the slit.Therefore, for example, when a slit opened in the outer periphery isformed in the honeycomb structure body, “the length of the current path”increases as much as a length of about twice a depth of the slit.

The electrical resistivity of the electrode 21 is preferably from 0.1 to100 Ωcm and further preferably from 0.1 to 50 Ωcm. When the electricalresistivity of the electrode 21 is in such a range, each of the pair ofelectrodes 21, 21 effectively performs the function as an electrode in apiping line where the exhaust gas of a high temperature flows. When theelectrical resistivity of the electrode 21 is smaller than 0.1 Ωcm, thetemperature of the honeycomb portion in the vicinity of each end of theelectrode 21 may easily rise, in the cross section perpendicular to thecell extending direction. When the electrical resistivity of theelectrode 21 is larger than 100 Ωcm, the current does not easily flow,and hence the function of the electrode may not easily be performed. Theelectrical resistivity of the electrode is a value at 400° C.

A porosity of the electrode 21 is preferably from 30 to 60% and furtherpreferably from 30 to 55%. When the porosity of the electrode 21 is insuch a range, a suitable electrical resistivity can be obtained. Whenthe porosity of the electrode 21 is lower than 30%, the electrode maydisadvantageously be deformed during manufacturing. When the porosity ofthe electrode 21 is higher than 60%, the electrical resistivity may beexcessively high. The porosity is a value measured by mercuryporosimeter.

An average pore diameter of the electrode 21 is preferably from 5 to 45μm and further preferably from 7 to 40 μm. When the average porediameter of the electrode 21 is in such a range, the suitable electricalresistivity can be obtained. When the average pore diameter of theelectrode 21 is smaller than 5 μm, the electrical resistivity may beexcessively high. When the average pore diameter of the electrode 21 islarger than 45 μm, the strength of the electrode 21 weakens and hencethe electrode may easily be damaged. The average pore diameter is avalue measured by mercury porosimeter.

When the main component of the electrode 21 is “silicon-silicon carbidecomposite material”, an average particle diameter of silicon carbideparticles contained in the electrode 21 is preferably from 10 to 60 μmand further preferably from 20 to 60 μm. When the average particlediameter of the silicon carbide particles contained in the electrode 21is in such a range, the electrical resistivity of the electrode 21 canbe controlled in the range of 0.1 to 100 Ωcm. When the average particlediameter of the silicon carbide particles contained in the electrode 21is smaller than 10 μm, the electrical resistivity of the electrode 21may be excessively large. When the average particle diameter of thesilicon carbide particles contained in the electrode 21 is larger than60 μm, the strength of the electrode 21 weakens and the electrode mayeasily be damaged. The average particle diameter of the silicon carbideparticles contained in the electrode 21 is a value measured by laserdiffraction method.

When the main component of the electrode 21 is “silicon-silicon carbidecomposite material”, a ratio of the mass of silicon contained in theelectrode 21 to “the total of respective masses of the silicon carbideparticles and silicon” contained in the electrode 21 is preferably from20 to 40 mass %. Furthermore, the ratio of the mass of silicon to “thetotal of the respective masses of the silicon carbide particles andsilicon” contained in the electrode 21 is further preferably from 25 to35 mass %. When the ratio of the mass of silicon to “the total of therespective masses of the silicon carbide particles and silicon”contained in the electrode 21 is in such a range, the electricalresistivity of the electrode 21 can be in a range of 0.1 to 100 Ωcm.When the ratio of the mass of silicon to “the total of the respectivemasses of the silicon carbide particles and silicon” contained in theelectrode 21 is smaller than 20 mass %, the electrical resistivity maybe excessively large. When the ratio is larger than 40 mass %, theelectrode may easily be deformed during the manufacturing.

An isostatic strength of the honeycomb structure 100 of the presentembodiment is preferably 1 MPa or more and further preferably 3 MPa ormore. A larger value of the isostatic strength is more preferable,however an upper limit of the value is about 6 MPa when the material,structure and the like of the honeycomb structure 100 are taken intoconsideration. When the isostatic strength is smaller than 1 MPa, thehoneycomb structure may easily be damaged during the use of thehoneycomb structure as a catalyst carrier. The isostatic strength is avalue measured under static pressure in water.

(2) First Honeycomb Structure Manufacturing Method:

Next, one embodiment of a first honeycomb structure manufacturing methodof the present invention will be described.

The one embodiment of the first honeycomb structure manufacturing methodof the present invention has a formed honeycomb body preparing step, adried honeycomb body preparing step, a fired honeycomb body preparingstep, a preparing step of the fired honeycomb body with unfiredelectrodes, and a honeycomb structure preparing step. Furthermore, inthe manufacturing method of the honeycomb structure of the presentembodiment, in the fired honeycomb body preparing step, a driedhoneycomb body is fired in a state where a plurality of particlescontaining, as a main component, cordierite, carbon or “a mixture ofthese components” are in contact with a side surface of the driedhoneycomb body. The formed honeycomb body preparing step is a step ofextrusion-forming a forming raw material containing a ceramic rawmaterial, to prepare a formed honeycomb body having partition walls todefine and form a plurality of cells which become through channels for afluid and extend from one end surface to the other end surface and anouter peripheral wall positioned in the most outer periphery. The driedhoneycomb body preparing step is a step of drying the formed honeycombbody to prepare the dried honeycomb body. The fired honeycomb bodypreparing step is a step of firing the dried honeycomb body to prepare afired honeycomb body. The preparing step of the fired honeycomb bodywith the unfired electrodes is a step in which an electrode forming rawmaterial containing a ceramic raw material is applied to a side surfaceof the fired honeycomb body and dried to form the unfired electrodes,thereby preparing the fired honeycomb body with the unfired electrodes.The honeycomb structure preparing step is a step of firing the firedhoneycomb body with the unfired electrodes to prepare the honeycombstructure.

As described above, in the manufacturing method of the honeycombstructure of the present embodiment, in the fired honeycomb bodypreparing step, the dried honeycomb body is fired in the state where theplurality of particles containing, as the main component “cordierite,carbon or “the mixture of these components”” are in contact with theside surface of the dried honeycomb body. Therefore, when the driedhoneycomb body is fired, silicon present in an outer peripheral regionof the dried honeycomb body is discharged to the outside by the presenceof “the plurality of particles containing, as the main component,“cordierite, carbon or a mixture of these components””, so that “thefired honeycomb body in which silicon of the outer peripheral regiondecreases” can be obtained. In consequence, the honeycomb structure tobe manufactured is the honeycomb structure having a central region. Themanufacturing method of the honeycomb structure of the presentembodiment is a manufacturing method in a case where the main componentof the honeycomb structure body is “a silicon-silicon carbide compositematerial”.

Hereinafter, the manufacturing method of the honeycomb structure of thepresent embodiment will be described step by step.

(2-1) Formed Honeycomb Body Preparing Step

First, metal silicon powder (metal silicon), a binder, a surfactant, apore former, water and the like are preferably added to silicon carbidepowder (silicon carbide) to prepare the forming raw material. Thesilicon carbide powder (silicon carbide) and the metal silicon powder(metal silicon) are ceramic raw materials. A mass of the metal siliconpowder to a total of a mass of the silicon carbide powder and the massof the metal silicon powder is preferably from 10 to 40 mass %. Anaverage particle diameter of silicon carbide particles in the siliconcarbide powder is preferably from 3 to 50 μm and further preferably from3 to 40 μm. An average particle diameter of metal silicon (the metalsilicon powder) is preferably from 2 to 35 μm. The average particlediameters of the silicon carbide particles and metal silicon (metalsilicon particles) are values measured by laser diffraction method. Thesilicon carbide particles are fine particles of silicon carbideconstituting the silicon carbide powder, and the metal silicon particlesare fine particles of metal silicon constituting the metal siliconpowder. It is to be noted that this is a composition of the forming rawmaterial in a case where the material of the honeycomb structure body isa silicon-silicon carbide composite material, and metal silicon is notadded in a case where the material of the honeycomb structure body issilicon carbide.

Examples of the binder include methylcellulose, hydroxypropylmethylcellulose, hydroxypropoxyl cellulose, hydroxyethylcellulose,carboxymethylcellulose, and polyvinyl alcohol. Among these examples,methylcellulose and hydroxypropoxyl cellulose are preferably usedtogether. A content of the binder is preferably from 2.0 to 10.0 partsby mass, when a total mass of the silicon carbide powder and the metalsilicon powder is 100 parts by mass.

A content of the water is preferably from 20 to 60 parts by mass, whenthe total mass of the silicon carbide powder and the metal siliconpowder is 100 parts by mass.

As the surfactant, ethylene glycol, dextrin, fatty acid soap,polyalcohol or the like can be used. One of these surfactants may beused alone, or a combination of two or more of the surfactants may beused. A content of the surfactant is preferably from 0.1 to 2.0 parts bymass, when the total mass of the silicon carbide powder and the metalsilicon powder is 100 parts by mass.

There is not any special restriction on the pore former as long as poresare formed after the firing, and examples of the pore former includegraphite, starch, foamable resin, a water-absorbable resin, and silicagel. A content of the pore former is preferably from 0.5 to 10.0 partsby mass, when the total mass of the silicon carbide powder and the metalsilicon powder is 100 parts by mass. An average particle diameter of thepore former is preferably from 10 to 30 μm. When the average particlediameter is smaller than 10 μm, the pores may not sufficiently beformed. When the average particle diameter is larger than 30 μm, the diemay be clogged during the forming. The average particle diameter of thepore former is a value measured by laser diffraction method.

Next, the forming raw material is preferably kneaded to form a kneadedclay. There is not any special restriction on a method of kneading theforming raw material to form the kneaded clay, and an example of themethod is a method using a kneader, a vacuum clay kneader or the like.

Next, the kneaded clay (forming raw material) is extrusion-formed toprepare the formed honeycomb body. During the extrusion-forming, it ispreferable to use a die having desirable entire shape, cell shape,partition wall thickness, cell density and the like. As a material ofthe die, a hard metal which does not easily wear down is preferable. Theformed honeycomb body is a structure having partition walls to defineand form a plurality of cells which become through channels for a fluidand extend from one end surface to the other end surface, and an outerperipheral wall positioned in the most outer periphery.

A partition wall thickness, a cell density, an outer peripheral wallthickness and the like of the formed honeycomb body can suitably bedetermined in accordance with a structure of the honeycomb structure ofthe present invention to be prepared, in consideration of shrinkages inthe drying and the firing.

(2-2) Dried Honeycomb Body Preparing Step

The obtained formed honeycomb body is dried. There is not any specialrestriction on a drying method, and examples of the method includeelectromagnetic heating systems such as microwave heating drying andhigh frequency dielectric heating drying, and external heating systemssuch as hot air drying and superheat steam drying. Among these, it ispreferable that a predetermined amount of a water content is dried bythe electromagnetic heating system and then the remaining water contentis dried by the external heating system, because the whole formed bodycan rapidly and evenly be dried so that cracks are not generated. Asdrying conditions, it is preferable that the water content of 30 to 99mass % of the amount of the water content prior to the drying is removedby the electromagnetic heating system and then the water content isdecreased to 3 mass % or less by the external heating system. As theelectromagnetic heating system, dielectric heating drying is preferable,and as the external heating system, the hot air drying is preferable. Adrying temperature is preferably from 50 to 100° C.

When a length of the formed honeycomb body in the central axis directionis not a desirable length, both end surfaces (both end portions) of theformed honeycomb body are preferably cut to obtain the desirable length.There is not any special restriction on a cutting method, but an exampleof the method is a method using a round saw cutter or the like.

(2-3) Fired Honeycomb Body Preparing Step

Next, the dried honeycomb body is fired to prepare the fired honeycombbody. Then, during the firing of the dried honeycomb body, the firing isperformed in a state where a plurality of particles (particles ofcordierite or the like) containing, as a main component, “cordierite,carbon or “a mixture of these components”” are in contact with a sidesurface of the dried honeycomb body. Here, examples of “the state wherethe particles of cordierite or the like are in contact with the sidesurface of the dried honeycomb body” include a case where the particlesof cordierite or the like directly come in contact with the side surfaceand a case where the particles are attached to the side surface in astate where other “particles of cordierite or the like” are interposedbetween the particles and the side surface. When the particles areattached to the side surface in the state where the other “particles ofcordierite or the like” are interposed between the particles and theside surface, it can be considered that the particles come in contactwith the side surface via the other “particles of cordierite or thelike”. Furthermore, the main component is a component to be contained asmuch as 90 mass % or more. Further specifically, it is preferable that“the particles of cordierite or the like” are dispersed in a dispersionmedium to prepare a dispersion liquid (slurry or paste), and thedispersion liquid is applied to the side surface (the outer peripheralshape) of the dried honeycomb body, followed by the firing. In “theparticles of cordierite or the like”, “cordierite, carbon or “themixture of these components”” are contained preferably as much as 70mass % or more, further preferably as much as 50 mass % or more, andespecially preferably as much as 80 mass % or more. In “the particles ofcordierite or the like”, cordierite is preferable. Furthermore, as adispersion medium, water is preferable.

Furthermore, when the honeycomb structure body is divided into theinflow side region and the outflow side region, an example of a methodof setting the electrical resistivity of the material constituting theinflow side region to be lower than the electrical resistivity of thematerial constituting the outflow side region includes the followingmethod. During the firing, “particles containing silicon as a maincomponent” are preferably laid on a mounting stand in such a manner thatgravels are laid, and the dried honeycomb body is preferably mountedthereon (on the particles containing silicon as the main component) byturning one end surface of the body downwardly. In consequence, duringthe firing, silicon of “the particles containing silicon as the maincomponent” is dissolved, and the dissolved silicon permeates the driedhoneycomb body from the one end surface thereof. Furthermore, in thefired honeycomb body obtained after the firing, the region permeated bysilicon becomes the inflow side region.

An amount of “the particles of cordierite or the like” to be in contactwith (attached to) the side surface of the dried honeycomb body ispreferably from 0.003 to 0.308 g/cm³, further preferably from 0.015 to0.154 g/cm³, and especially preferably from 0.022 to 0.123 g/cm³. Whenthe amount is smaller than 0.003 g/cm³, the electrical resistivity ofthe outer peripheral region of the honeycomb structure may not easily beraised. When the amount is larger than 0.308 g/cm³, the electricalresistivity of the outer peripheral region of the honeycomb structuremay be excessively high. The above-mentioned unit of “g/cm³” indicatesgrams (g) per unit area (cm³) of the side surface of the dried honeycombbody.

A content ratio of “the particles of cordierite or the like” in thedispersion liquid is preferably 50 mass % or more, further preferably 70mass % or more, and especially preferably 80 mass % or more. When thecontent ratio is lower than 50 mass %, cracks may be generated in thishoneycomb body due to movement of water content during the drying. Theaverage particle diameter of “the particles of cordierite or the like”is preferably from 0.1 to 10 μm, further preferably from 1 to 7 μm, andespecially preferably from 3 to 7 μm. When the average particle diameteris smaller than 0.1 μm, the particles secondarily aggregate in thedispersion liquid, and it is difficult to evenly prepare the dispersionliquid. When the average particle diameter is larger than 10 μm, theelectrical resistivity of the outer peripheral region may be locallyexcessively high. The average particle diameter is a value measured bylaser diffraction method.

Prior to the firing, calcination is preferably performed to remove thebinder and the like. The calcination is preferably performed at 400 to500° C. in the air atmosphere for 0.5 to 20 hours. There is not anyspecial restriction on a calcination and firing method, and the firingcan be performed by using an electric furnace, a gas furnace or thelike. As firing conditions, heating is preferably performed at 1400 to1500° C. in an inert atmosphere of nitrogen, argon or the like for oneto 20 hours. Furthermore, after the firing, for enhancement of adurability, an oxygenation treatment is preferably performed at 1200 to1350° C. for one to ten hours.

(2-4) Preparing Step of Fired Honeycomb Body with Unfired Electrodes

Next, an electrode forming raw material to form electrodes is preferablyblended. When a main component of the electrodes is “a silicon-siliconcarbide composite material”, the electrode forming raw material ispreferably formed by adding predetermined additives to silicon carbidepowder and silicon powder, followed by the kneading.

Specifically, metal silicon powder (metal silicon), a binder, asurfactant, a pore former, water and the like are preferably added tothe silicon carbide powder (silicon carbide) and kneaded to prepare theelectrode forming raw material. When a total mass of the silicon carbidepowder and metal silicon is 100 parts by mass, the mass of metal siliconis preferably from 20 to 40 parts by mass. An average particle diameterof silicon carbide particles in the silicon carbide powder is preferablyfrom 10 to 60 μm. An average particle diameter of the metal siliconpowder (metal silicon) is preferably from 2 to 20 μm. When the averageparticle diameter of the metal silicon powder (metal silicon) is smallerthan 2 μm, the electrical resistivity may be excessively small. When theaverage particle diameter of the metal silicon powder (metal silicon) islarger than 20 μm, the electrical resistivity may be excessively large.The average particle diameters of the silicon carbide particles andmetal silicon (metal silicon particles) are values measured by laserdiffraction method. The silicon carbide particles are fine particles ofsilicon carbide constituting the silicon carbide powder, and the metalsilicon particles are fine particles of metal silicon constituting themetal silicon powder.

Examples of the binder include methylcellulose, hydroxypropylmethylcellulose, hydroxypropoxyl cellulose, hydroxyethylcellulose,carboxymethylcellulose, and polyvinyl alcohol. Among these examples,methylcellulose and hydroxypropoxyl cellulose are preferably usedtogether. A content of the binder is preferably from 0.1 to 5.0 parts bymass, when a total mass of the silicon carbide powder and the metalsilicon powder is 100 parts by mass.

A content of the water is preferably from 15 to 60 parts by mass, whenthe total mass of the silicon carbide powder and the metal siliconpowder is 100 parts by mass.

As the surfactant, ethylene glycol, dextrin, fatty acid soap,polyalcohol or the like can be used. One of these surfactants may beused alone, or a combination of two or more of the surfactants may beused. A content of the surfactant is preferably from 0.1 to 2.0 parts bymass, when the total mass of the silicon carbide powder and the metalsilicon powder is 100 parts by mass.

There is not any special restriction on the pore former as long as poresare formed after the firing, and examples of the pore former includegraphite, starch, foamable resin, a water-absorbable resin, and silicagel. A content of the pore former is preferably from 0.1 to 5.0 parts bymass, when the total mass of the silicon carbide powder and the metalsilicon powder is 100 parts by mass. An average particle diameter of thepore former is preferably from 10 to 30 μm. When the average particlediameter is smaller than 10 μm, the pores may not sufficiently beformed. When the average particle diameter is larger than 30 μm, largepores are easily formed, and hence strength deterioration may occur. Theaverage particle diameter of the pore former is a value measured bylaser diffraction method.

Next, a mixture obtained by mixing the silicon carbide powder (siliconcarbide), metal silicon (the metal silicon powder), the binder, thesurfactant, the pore former, the water and the like is preferablykneaded, to obtain the paste-like or slurry-like electrode forming rawmaterial. There is not any special restriction on a kneading method and,for example, a vertical stirrer can be used.

Next, the obtained “electrode forming raw material containing theceramic raw material” is preferably applied to the side surface of thefired honeycomb body. There is not any special restriction on a methodof applying the electrode forming raw material to the side surface ofthe fired honeycomb body but, for example, a printing method can beused. Furthermore, the electrode forming raw material is preferablyapplied to the side surface of the fired honeycomb body so as to obtainthe above-mentioned shape of the electrodes in the honeycomb structureof the present invention. A thickness of each electrode can be set to adesirable thickness by regulating the thickness of the electrode formingraw material during the application thereof. As described above, theelectrodes can be formed simply by applying the electrode forming rawmaterial to the side surface of the fired honeycomb body, followed bythe drying and the firing, and hence the electrodes can very easily beformed.

Next, the electrode forming raw material applied to the side surface ofthe fired honeycomb body is preferably dried to form unfired electrodes,thereby preparing the fired honeycomb body with the unfired electrodes.A drying temperature as a drying condition is preferably from 50 to 100°C.

(2-5) Honeycomb Structure Preparing Step

Next, the fired honeycomb body with the unfired electrodes are fired toprepare a honeycomb structure. At this time, the unfired electrodes aremainly fired. Prior to the firing, the calcination is preferablyperformed to remove the binder and the like. The calcination ispreferably performed at 400 to 500° C. in the air atmosphere for 0.5 to20 hours. There is not any special restriction on a calcination andfiring method, and the firing can be performed by using an electricfurnace, a gas furnace or the like. As firing conditions, the heating ispreferably performed at 1400 to 1500° C. in an inert atmosphere ofnitrogen, argon or the like for one to 20 hours. Furthermore, after thefiring, for the enhancement of the durability, an oxygenation treatmentis preferably performed at 1200 to 1350° C. for one to ten hours.

(3) Second Honeycomb Structure Manufacturing Method:

Next, one embodiment of a second honeycomb structure manufacturingmethod of the present invention will be described.

The one embodiment of the second honeycomb structure manufacturingmethod of the present invention has a formed honeycomb body preparingstep, a dried honeycomb body preparing step, a preparing step of thedried honeycomb body with unfired electrodes, and a honeycomb structurepreparing step. The formed honeycomb body preparing step is a step ofextrusion-forming a forming raw material containing a ceramic rawmaterial, to prepare a formed honeycomb body having partition walls todefine and form a plurality of cells which become through channels for afluid and extend from one end surface to the other end surface and anouter peripheral wall positioned in the most outer periphery. The driedhoneycomb body preparing step is a step of drying the formed honeycombbody to prepare a dried honeycomb body. The preparing step of the driedhoneycomb body with unfired electrodes is a step in which an electrodeforming raw material containing a ceramic raw material is applied to aside surface of the dried honeycomb body and dried to form the unfiredelectrodes, thereby preparing the dried honeycomb body with the unfiredelectrodes. The honeycomb structure preparing step is a step of firingthe dried honeycomb body with the unfired electrodes to prepare thehoneycomb structure. Furthermore, in the honeycomb structure preparingstep, the dried honeycomb body with the unfired electrodes is fired in astate where a plurality of particles (specific particles) containing, asa main component, cordierite, carbon or a mixture of these componentsare in contact with the side surface of the dried honeycomb body withthe unfired electrodes.

As described above, in the manufacturing method of the honeycombstructure of the present embodiment, in the honeycomb structurepreparing step, the dried honeycomb body with the unfired electrodes isfired in the state where “the specific particles” are in contact withthe side surface of the dried honeycomb body with the unfiredelectrodes. Therefore, when the dried honeycomb body with the unfiredelectrodes is fired, silicon present in the outer peripheral region ofthe dried honeycomb body with the unfired electrodes is discharged tothe outside by the presence of “the specific particles”, so that “thehoneycomb structure in which silicon in the outer peripheral region isdecreased” can be obtained. In consequence, the honeycomb structure tobe manufactured is a honeycomb structure having a central region. Themanufacturing method of the honeycomb structure of the presentembodiment is a manufacturing method in a case where the main componentof the honeycomb structure body is “a silicon-silicon carbide compositematerial”.

Hereinafter, the manufacturing method of the honeycomb structure of thepresent embodiment will be described step by step.

(3-1) Formed Honeycomb Body Preparing Step

The formed honeycomb body preparing step is preferably similar to “theformed honeycomb body preparing step” in the above-mentioned firsthoneycomb structure manufacturing method of the present invention.

(3-2) Dried Honeycomb Body Preparing Step

The dried honeycomb body preparing step is preferably similar to “thedried honeycomb body preparing step” in the above-mentioned firsthoneycomb structure manufacturing method of the present invention.

(3-3) Preparing Step of Dried Honeycomb Body with Unfired Electrodes

The preparing step of the dried honeycomb body with the unfiredelectrodes is preferably “the preparing step of the fired honeycomb bodywith the unfired electrodes” in the above-mentioned first honeycombstructure manufacturing method of the present invention, wherein “thefired honeycomb body” is replaced with “the dried honeycomb body”.

(3-4) Honeycomb Structure Preparing Step

The honeycomb structure preparing step is preferably “the firedhoneycomb body preparing step” in the above-mentioned first honeycombstructure manufacturing method of the present invention, wherein “thedried honeycomb body” is replaced with “the dried honeycomb body withthe unfired electrodes”.

In the manufacturing method of the honeycomb structure of the presentembodiment, the number of times of the firing is one only in thehoneycomb structure preparing step, and hence there is an advantage thatthe production efficiency is high. On the contrary, in theabove-mentioned first honeycomb structure manufacturing method of thepresent invention, there is an advantage that “the specific particles”can more evenly be attached to the side surface of the dried honeycombbody.

In the manufacturing method of the honeycomb structure of the presentembodiment, when “the specific particles” are in contact with the sidesurface of the dried honeycomb body with the unfired electrodes, “thespecific particles” are preferably in contact with (attached to) aportion of the side surface of the dried honeycomb body on which theunfired electrodes are not disposed.

EXAMPLES

Hereinafter, examples of the present invention will further specificallybe described, but the present invention is not limited to theseexamples.

Example 1

Silicon carbide (SiC) powder and metal silicon (Si) powder were mixed ata mass ratio of 80:20 to prepare a ceramic raw material. Then, to theceramic raw material, hydroxypropyl methylcellulose as a binder and awater-absorbable resin as a pore former were added, and water was alsoadded to prepare a forming raw material. Then, the forming raw materialwas kneaded by a vacuum clay kneader to prepare a columnar kneaded clay.The content of the binder was 7 parts by mass when a total of thesilicon carbide (SiC) powder and the metal silicon (Si) powder was 100parts by mass. The content of the pore former was 3 parts by mass whenthe total of the silicon carbide (SiC) powder and the metal silicon (Si)powder was 100 parts by mass. The content of the water was 42 parts bymass when the total of the silicon carbide (SiC) powder and the metalsilicon (Si) powder was 100 parts by mass. The average particle diameterof the silicon carbide powder was 20 μm, and the average particlediameter of the metal silicon powder was 6 μm. Furthermore, the averageparticle diameter of the pore former was 20 μm. The average particlediameters of silicon carbide, metal silicon and pore former are valuesmeasured by laser diffraction method.

The obtained columnar kneaded clay was extruded by using anextrusion-forming machine, to obtain a formed honeycomb body. Theobtained formed honeycomb body was dried by high frequency dielectricheating, and then dried at 120° C. for two hours by use of a hot airdryer, and both end surfaces of the formed honeycomb body were cut asmuch as a predetermined amount, to prepare a dried honeycomb body.

Next, a slurry (a dispersion liquid) in which cordierite powder wasdispersed in water was applied to a side surface of the dried honeycombbody. The concentration of the cordierite powder in the dispersionliquid was 0.2 mass %. Moreover, the average particle diameter of thecordierite powder was 5 μm. The average particle diameter is a valuemeasured by laser diffraction method. The amount of the cordieritepowder attached to “the side surface of the dried honeycomb body” was0.1 g/cm³ per unit area of the side surface of the dried honeycomb body.

Afterward, the dried honeycomb body to which the dispersion liquid wasattached was degreased (calcinated), fired and further subjected to anoxidation treatment to obtain a fired honeycomb body. The degreasingcondition was three hours at 550° C. The firing condition was two hoursin an argon atmosphere at 1450° C. The condition of the oxidationtreatment was one hour at 1300° C.

Next, silicon carbide (SiC) powder and metal silicon (Si) powder weremixed at a mass ratio of 60:40, and to this mixture, hydroxypropylmethylcellulose as a binder, glycerin as a moisture retaining agent anda surfactant as a dispersant were added, and water was also added,followed by mixing. The mixture was kneaded to prepare an electrodeforming raw material. The content of the binder was 0.5 part by masswhen the total of the silicon carbide (SiC) powder and the metal silicon(Si) powder was 100 parts by mass. The content of glycerin was 10 partsby mass when the total of the silicon carbide (SiC) powder and the metalsilicon (Si) powder was 100 parts by mass. The content of the surfactantwas 0.3 part by mass when the total of the silicon carbide (SiC) powderand the metal silicon (Si) powder was 100 parts by mass. The content ofwater was 42 parts by mass when the total of the silicon carbide (SiC)powder and the metal silicon (Si) powder was 100 parts by mass. Theaverage particle diameter of the silicon carbide powder was 52 μm, andthe average particle diameter of the metal silicon powder was 6 μm. Theaverage particle diameters of silicon carbide and metal silicon arevalues measured by laser diffraction method. The kneading was performedby using a vertical stirrer.

Next, the electrode forming raw material was applied to the side surfaceof the fired honeycomb body in a band-like manner to extend between bothend surfaces of the fired honeycomb body so that a thickness was 0.15 mmand “an angle of 0.5 times a central angle in a cross sectionperpendicular to a cell extending direction was 50°”. The electrodeforming raw material was applied to two portions of the side surface ofthe fired honeycomb body. Then, in the cross section perpendicular tothe cell extending direction, one of the two portions to which theelectrode forming raw material was applied was disposed on a sideopposite to the other portion via a center of the fired honeycomb body.

Next, the electrode forming raw material applied to the fired honeycombbody was dried, to obtain the fired honeycomb body with unfiredelectrodes. The drying temperature was 70° C.

Afterward, the fired honeycomb body with the unfired electrodes wasdegreased (calcinated), fired and further subjected to an oxidationtreatment to obtain a honeycomb structure. The degreasing condition wasthree hours at 550° C. The firing condition was two hours in an argonatmosphere at 1450° C. The condition of the oxidation treatment was onehour at 1300° C.

The average pore diameter (pore diameters) of partition walls of theobtained honeycomb structure was 8.6 μm, and the porosity was 45%. Theaverage pore diameter and the porosity are values measured by mercuryporosimeter. Furthermore, the honeycomb structure had a partition wallthickness of 90 μm and a cell density of 90 cells/cm². Furthermore, thebottom surface of the honeycomb structure had a round shape with adiameter of 93 mm, and a length of the honeycomb structure in the cellextending direction was 75 mm. Furthermore, the isostatic strength ofthe obtained honeycomb structure was 2.5 MPa. The isostatic strength isa breaking strength measured under static pressure in water.Furthermore, the angle of 0.5 times the central angle of each of twoelectrodes in the cross section of the honeycomb structure which wasperpendicular to the cell extending direction was 50°. Furthermore, thethickness of each of the two electrodes was 1.5 mm. Furthermore, theelectrical resistivity of the electrodes was 1.3 Ωcm, the electricalresistivity of the central region of the honeycomb structure body was 35Ωcm, and the electrical resistivity of the outer peripheral region was50 Ωcm. The radius of the cross section of the central region which wasperpendicular to the cell extending direction (central region radius)was 25 mm. The central region radius is a distance from the center O tothe outer periphery of the central region in the cross sectionperpendicular to the cell extending direction. Furthermore, the centralregion had a boundary region. The thickness of the boundary region was10 mm. The electrical resistivity of the boundary region graduallylowered from “the end on the side of the central region” to “theboundary with the outer peripheral region”. The electrical resistivitiesare values measured by four-terminal method.

Additionally, the electrical resistivity of each of the central regionand the outer peripheral region of the honeycomb structure body and that“the electrical resistivity of the boundary region gradually changed”were observed by the following method. The honeycomb structure body wascut into round slices so as to form a disc (disc 41, refer to FIG. 6)having a cross section perpendicular to the cell extending direction andhaving a thickness of 1 cm. The discs were cut out from three portionsof “both end portions and the central portion” of the honeycombstructure body in “the cell extending direction”. Then, as shown in FIG.6, each of the discs 41 was cut into each width of 3 mm along a straightline passing the center of “the end surface of the disc 41” and straightlines parallel to the straight line, to prepare a plurality of rod-likesamples 42 as shown in FIG. 7. In FIG. 6, “the straight line passing thecenter of the end surface of the disc 41 and the straight lines parallelto the straight line” are shown by broken lines. Furthermore, theabove-mentioned “width of 3 mm” means that the distance between thestraight lines shown by the broken lines in FIG. 6 is 3 mm.

Then, as shown in FIG. 7, a voltage was applied to each of the rod-likesamples 42, and a volume resistivity (electrical resistivity) of a rangeof a length L in the central portion was obtained (4-lines resistancetype measurement). The length L was 4 cm. The rod-like samples 42 cutout from “the same position in each of the three discs 41” are to be“the rod-like samples of the same position”. That is, three “rod-likesamples of the same position” are present (one sample can be taken fromeach disc). Then, an average value of the electrical resistivities ofthe three “rod-like samples of the same position” was taken, to obtainthe electrical resistivity of the position. In consequence, theelectrical resistivities of the central region and the outer peripheralregion of the honeycomb structure body were confirmed, and it wasconfirmed that the electrical resistivity of the boundary regiongradually changed. FIG. 6 shows the disc 41 formed by cutting thehoneycomb structure of Example 1 into the round slices. FIG. 7 shows therod-like sample 42 cut out from the honeycomb structure of Example 1.

Furthermore, the electrical resistivity of each electrode was measuredby the following method. A test piece of 10 mm×10 mm×50 mm was preparedby using the same material as the electrodes. A silver paste was appliedto the whole surfaces of both end portions of the test piece, so that itwas possible to energize the test piece via a wiring line. The testpiece was connected to a voltage applying current measuring device toapply a voltage to the test piece. A thermocouple was disposed in acentral region of the test piece, and a change of a temperature of thetest piece with an elapse of time during the application of the voltagewas confirmed by a recorder. A voltage of 100 to 200 V was applied, acurrent value and a voltage value were measured in a state where thetest piece temperature was 400° C., and the electrical resistivity wascalculated from the obtained current value and voltage value and thetest piece dimension.

As to the obtained honeycomb structure, “the purification performance”was measured by the following method. The results are shown in Table 1.

(Purification Performance)

The honeycomb structure was contained in a cylindrical containingcontainer, and the container was attached to an exhaust pipe of agasoline engine with a displacement of 2.0 L. Running was carried out inan exhaust gas regulation mode “JC-08”, and the exhaust gas was sampledfrom the piping line connected to the exhaust pipe and stored in a bag.The piping line for the sampling was disposed on a downstream side ofthe container in which the honeycomb structure was contained. After theend of the running, the exhaust gas stored in the bag was analyzed, andHC emission was measured (in conformity to the stipulation of JC-08).The value of the ratio of an inverse number of the HC emission to thevalue (inverse number of the HC emission) in Comparative Example 1 isthe value of the purification performance. When the value of thepurification performance is larger than 1, it indicates that thepurification performance is suitable as compared with ComparativeExample 1. In the measurement of the purification performance, at coldstart during the running in the exhaust gas regulation mode “JC-08”, avoltage is applied to the honeycomb structure at 6 kW for 20 seconds.

TABLE 1 Radius of Thickness of Electrical resistivity of Electricalresistivity of central region boundary region central region outerperipheral region Purification (mm) (mm) (Ωcm) (Ωcm) performance Example1 25 10 35 50 1.31 Example 2 30 10 35 50 1.34 Example 3 35 10 35 50 1.37Example 4 25 5 35 50 1.28 Example 5 25 15 35 50 1.33 Example 6 25 10 3050 1.43 Example 7 25 10 25 50 1.75 Example 8 25 10 35 40 1.15 Example 925 10 35 60 1.53 Comparative 0 0 Electrical resistivity of honeycombstructure 1.00 Example 1 body 35

Examples 2 to 9 and Comparative Example 1

The procedures of Example 1 were repeated except that electricalresistivities (of materials) of the central region and the outerperipheral region, the radius of the central region (radius in a crosssection perpendicular to a cell extending direction) and the thicknessof the boundary region were changed as shown in Table 1, to preparehoneycomb structures. In the same manner as in Example 1, “thepurification performance” of each of the honeycomb structures wasmeasured. The results are shown in Table 1.

It can be seen from Table 1 that when the electrical resistivity of thematerial constituting the central region is lower than the electricalresistivity of the material constituting the outer peripheral region,the purification efficiency per unit amount of thrown energy issuitable. That is, it is seen that the amount of the energy for use toobtain the equivalent purification efficiency may be small.

The honeycomb structure of the present invention can suitably beutilized as a catalyst carrier for an exhaust gas purifying device whichpurifies an exhaust gas of a car.

DESCRIPTION OF REFERENCE SYMBOLS

1: partition wall, 2: cell, 3: outer peripheral wall, 4: honeycombstructure body, 5: side surface, 6: central region, 7: outer peripheralregion, 8: boundary region, 8 a: end on the side of the central region,8 b: boundary with the outer peripheral region, 11: inflow end surface,12: outflow end surface, 21: electrode, 41: disc, 42: rod-like sample,100 and 200: honeycomb structure, O: center, α: central angle, β: angle,θ: angle of 0.5 times the central angle, and L: length.

What is claimed is:
 1. A honeycomb structure comprising: a tubularhoneycomb structure body having porous partition walls to define andform a plurality of cells which become through channels for a fluid andextend from an inflow end surface which is an end surface on an inflowside of the fluid to an outflow end surface which is an end surface onan outflow side of the fluid, and an outer peripheral wall positioned inthe outermost periphery; and a pair of electrodes disposed on a sidesurface of the tubular honeycomb structure body, wherein an electricalresistivity of the tubular honeycomb structure body is from 1 to 200Ωcm, each of the pair of electrodes is formed into a band-like shapeextending in an extending direction of the plurality of cells of thetubular honeycomb structure body, and in a cross section perpendicularto the extending direction of the plurality of cells, one electrode inthe pair of electrodes is disposed on a side opposite to the otherelectrode in the pair of electrodes via a center of the tubularhoneycomb structure body, the tubular honeycomb structure body isconstituted of an outer peripheral region including a side surface and acentral region as a region around the center which excludes the outerperipheral region, the outer peripheral region completely surroundingthe central region, and an electrical resistivity of the central regionis lower than an electrical resistivity of the outer peripheral region.2. The honeycomb structure according to claim 1, wherein an electricalresistivity of a material constituting the central region is lower thanan electrical resistivity of a material constituting the outerperipheral region.
 3. The honeycomb structure according to claim 1,wherein the tubular honeycomb structure body and the pair of electrodesare made of a material including silicon carbide.
 4. The honeycombstructure according to claim 2, wherein the tubular honeycomb structurebody and the pair of electrodes are made of a material including siliconcarbide.
 5. The honeycomb structure according to claim 1, wherein in thecross section perpendicular to the extending direction of the pluralityof cells, a length of a current path is 1.6 times or less a diameter ofthe tubular honeycomb structure body.
 6. The honeycomb structureaccording to claim 2, wherein in the cross section perpendicular to theextending direction of the plurality of cells, a length of a currentpath is 1.6 times or less a diameter of the tubular honeycomb structurebody.
 7. The honeycomb structure according to claim 3, wherein in thecross section perpendicular to the extending direction of the pluralityof cells, a length of a current path is 1.6 times or less a diameter ofthe tubular honeycomb structure body.
 8. The honeycomb structureaccording to claim 4, wherein in the cross section perpendicular to theextending direction of the plurality of cells, a length of a currentpath is 1.6 times or less a diameter of the tubular honeycomb structurebody.
 9. The honeycomb structure according to claim 1, wherein thecentral region has a boundary region in a boundary portion between thecentral region and the outer peripheral region, and the boundary regionis a region where the electrical resistivity gradually changes so thatthe electrical resistivity lowers toward the closer boundary portion tothe outer peripheral region.
 10. The honeycomb structure according toclaim 2, wherein the central region has a boundary region in a boundaryportion between the central region and the outer peripheral region, andthe boundary region is a region where the electrical resistivitygradually changes so that the electrical resistivity lowers toward thecloser boundary portion to the outer peripheral region.
 11. Thehoneycomb structure according to claim 3, wherein the central region hasa boundary region in a boundary portion between the central region andthe outer peripheral region, and the boundary region is a region wherethe electrical resistivity gradually changes so that the electricalresistivity lowers toward the closer boundary portion to the outerperipheral region.
 12. The honeycomb structure according to claim 4,wherein the central region has a boundary region in a boundary portionbetween the central region and the outer peripheral region, and theboundary region is a region where the electrical resistivity graduallychanges so that the electrical resistivity lowers toward the closerboundary portion to the outer peripheral region.
 13. The honeycombstructure according to claim 5, wherein the central region has aboundary region in a boundary portion between the central region and theouter peripheral region, and the boundary region is a region where theelectrical resistivity gradually changes so that the electricalresistivity lowers toward the closer boundary portion to the outerperipheral region.
 14. The honeycomb structure according to claim 6,wherein the central region has a boundary region in a boundary portionbetween the central region and the outer peripheral region, and theboundary region is a region where the electrical resistivity graduallychanges so that the electrical resistivity lowers toward the closerboundary portion to the outer peripheral region.
 15. The honeycombstructure according to claim 7, wherein the central region has aboundary region in a boundary portion between the central region and theouter peripheral region, and the boundary region is a region where theelectrical resistivity gradually changes so that the electricalresistivity lowers toward the closer boundary portion to the outerperipheral region.
 16. The honeycomb structure according to claim 8,wherein the central region has a boundary region in a boundary portionbetween the central region and the outer peripheral region, and theboundary region is a region where the electrical resistivity graduallychanges so that the electrical resistivity lowers toward the closerboundary portion to the outer peripheral region.
 17. A manufacturingmethod of a honeycomb structure having: a formed tubular honeycombstructure body preparing step of extrusion-forming a forming rawmaterial containing a ceramic raw material, to prepare a formed tubularhoneycomb structure body having partition walls to define and form aplurality of cells which become through channels for a fluid and extendfrom one end surface to the other end surface and an outer peripheralwall positioned in the outermost periphery; a dried tubular honeycombstructure body preparing step of drying the formed tubular honeycombstructure body to prepare a dried tubular honeycomb structure body; afired tubular honeycomb structure body preparing step of firing thedried tubular honeycomb structure body to prepare a fired tubularhoneycomb structure body; a preparing step of the fired tubularhoneycomb structure body with a pair of unfired electrodes in which anelectrode forming raw material containing a ceramic raw material isapplied to a side surface of the fired tubular honeycomb structure bodyand dried to form the pair of unfired electrodes, thereby preparing thefired tubular honeycomb structure body with the pair of unfiredelectrodes; and a tubular honeycomb structure body preparing step offiring the fired tubular honeycomb structure body with the the pair ofunfired electrodes to prepare the tubular honeycomb structure body,wherein in the fired tubular honeycomb structure body preparing step,the dried tubular honeycomb structure body is fired in a state where aplurality of particles containing, as a main component, cordierite,carbon or a mixture of these components are in contact with a sidesurface of the dried tubular honeycomb structure body, an electricalresistivity of the tubular honeycomb structure body is from 1 to 200Ωcm, each of the pair of electrodes is formed into a band-like shapeextending in an extending direction of the plurality of cells of thetubular honeycomb structure body, and in a cross section perpendicularto the extending direction of the plurality of cells, one electrode inthe pair of electrodes is disposed on a side opposite to the oilierelectrode in the pair of electrodes via a center of the tubularhoneycomb structure body, the tubular honeycomb structure body isconstituted of an outer peripheral region including a side surface and acentral region as a region around the center which excludes the outerperipheral region, the outer peripheral region completely surroundingthe central region, and an electrical resistivity of the central re ionis lower than an electrical resistivity of the outer peripheral region.18. A manufacturing method of a honeycomb structure having: a formedtubular honeycomb structure body preparing step of extrusion-forming aforming raw material containing a ceramic raw material, to prepare aformed tubular honeycomb structure body having partition walls to defineand form a plurality of cells which become through channels for a fluidand extend from one end surface to the other end surface and an outerperipheral wall positioned in the outermost periphery; a dried tubularhoneycomb structure body preparing step of drying the formed tubularhoneycomb structure body to prepare a dried tubular honeycomb structurebody; a preparing step of the dried tubular honeycomb structure bodywith a pair of unfired electrodes in which an electrode forming rawmaterial containing a ceramic raw material is applied to a side surfaceof the dried tubular honeycomb structure body and dried to form the pairof unfired electrodes, thereby preparing the dried tubular honeycombstructure body with the pair of unfired electrodes; and a tubularhoneycomb structure body preparing step of firing the dried tubularhoneycomb structure body with the pair of unfired electrodes to preparethe tubular honeycomb structure body, wherein in the tubular honeycombstructure body preparing step, the dried tubular honeycomb structurebody with the pair of unfired electrodes is fired in a state where aplurality of particles containing, as a main component, cordierite,carbon or a mixture of these components are in contact with the sidesurface of the dried tubular honeycomb structure body with the pair ofunfired electrodes, an electrical resistivity of the tubular honeycombstructure body is from 1 to 200 Ωcm, each of the pair of electrodes isformed into a band-like shape extending in an extending direction of theplurality of cells of the tubular honeycomb structure body, and in across section perpendicular to the extending direction of the pluralityof cells, one electrode in the pair of electrodes is disposed on a sideopposite to the other electrode in the pair of electrodes via a centerof the tubular honeycomb structure body the tubular honeycomb structurebody is constituted of an outer peripheral region including a sidesurface and a central region as a region around the center whichexcludes the outer peripheral region, the outer peripheral regioncompletely surrounding the central region, and an electrical resistivityof the central region is lower than an electrical resistivity of theouter peripheral region.